All fiber lines include some type of washer in order to separate digestion liquor from the pulp. Later on in the process a washing arrangement is provided to separate bleaching liquors after bleaching stages. There exist several different types of washing arrangements operating according to different principles.
One type of washing arrangement is the drum washer where the pulp is dewatered on a rotary filter drum after the addition of washing liquid, which displaces the liquor remaining on the pulp web after preceding processing stages, for example a digestion or bleaching stage. The static pressure causes the displaced liquor to pass through perforated metal sheet mounted on the rotary drum. A further development of the original drum washer is the pressurized displacement washer where the filtrate at overpressure, is caused to pass through the metal sheet. The increased pressure difference effects an improved dewatering of the pulp. In the pressurized displacement washer the increased pressure difference can cause the pulp web to deposit itself harder on the metal sheet of the drum and at times must be removed by some kind of auxiliary means. The pulp web, for example, then can be loosened by means of liquid or air.
According to a known design of a pressurized displacement washer, the drum is provided with compartments, in which the pulp places itself in the form of rectangles, oval in the axial direction of the drum against the metal sheet. The compartmentalization of the drum ensures that the pulp cake does not break up and starts moving, but instead maintains the form brought about at the deposition of the pulp. The compartments consist of bars placed axially along the entire axle of the drum, which bars are the walls of the compartments. The perforated metal sheet, on which the pulp deposits, is located spaced from the drum, so that filtrate channels are formed in the space between the drum and the sheet. Along the circumference of the drum there are, thus, at least as many filtrate compartments as pulp compartments. In a drum washer a plurality of different washing stages can be carried out, with separate addition of washing liquid to the different stages, and also recycling of filtrate from one stage as washing liquid to another stage.
In order to maintain maximum washing effectiveness, it is desirable to ensure that washing liquid intended for a specific washing stage is not moved to a later washing stage. Washing liquid intended for a washing stage later in the process is cleaner than washing liquid used in a preceding washing stage. A difference in pressure between the stages causes added washing liquid to tend to move to the lower pressure. In order to be able to separate different washing stages as well as forming stages and discharging stages, the respective zones are sealed by axial seals, which are placed between the compartment walls of the rotary drum and the surrounding pressure-bearing casing.
In order to increase the effectiveness of a washing apparatus, it can be designed so that the washing liquid is caused to move in a countercurrent flow through the pulp. It is thereby intended that filtrate from a washing stage is recycled as washing liquid to a preceding washing stage. In cases when the washing arrangement is of the type compartmented drum, often a peripheral valve is mounted on one or both of the end walls of the drum, in order to collect filtrate from the filtrate channels. The filtrate, thus, is moved in axial direction of the drum outward to one or both of the drum end walls. In order to separate the filtrate from the different washing stages, the valve is provided with seals, which define different parts in the valve. The filtrate is then pumped on to a preceding washing stage alternatively to a filtrate tank. The seals of the valve are placed so that they are in line with the seals defining each washing stage. In this way all filtrate from a washing stage will be collected within the same area in the peripheral valve.
It has been found, however, that the division of filtrate does not work satisfactorily. In a washing zone there is space for several compartments. The filtrate in the channels of the compartments located at the end of the washing zone lands on the wrong side of the seal placed in the valve. In this way part of the filtrate from a washing stage will be mixed with filtrate from subsequent washing stages. As the filtrate from the next following washing stage is cleaner, this filtrate is slightly contaminated. When then the filtrate from the subsequent washing stage is then re-used as washing liquid in a preceding washing stage, the washing effectiveness is deteriorated. In order to reduce this effect, the channels have been made more shallow, so that the volume is decreased and consequently not as much of the filtrate can be transferred. The shallow channels, however, give rise to high pressure drops in the channels, which causes capacity and effectiveness problems.
One object of the present invention is to eliminate or at least reduce the aforesaid problems.